Various electronic devices have audio driver, i.e. amplifier, circuitry for driving an audio signal to an audio load, e.g. a loudspeaker, and often devices may have a connector, in the form of a plug and socket for example, for outputting audio signals to an external device. The external device could be a peripheral device such as a headset with relatively small loudspeakers, e.g. located in earphones or earbuds or the like. Such headset loudspeakers will typically have a relatively low impedance, say of the order of a few ohms or so. Additionally however it is often desired that the same audio driver circuitry can be used to drive higher impedance loudspeakers when connected, say of the order of a few hundred ohms or so. It is also desirable for the same audio driver circuitry to be able to drive a line load of the order of a few kilo-ohms or more via the audio connector, e.g. when connected to a suitable audio system, which may for instance be part of a docking arrangement. Thus the audio driver circuitry should be capable of driving different types of load.
For high quality sound reproduction the distortion of the audio signal, especially THD (total harmonic distortion) and intermodulation distortion, should be low. Various factors may contribute to distortion in an amplifier circuit driving a load. Typically the distortion is worst for lower impedance loads. An amplifier may be designed to maintain adequately low distortion into a low impedance load by increasing the output stage quiescent current, generally requiring larger output devices and higher current pre-driver stages. Distortion may also be managed by designing the amplifier circuitry with multiple amplifier stages to attempt to suppress output distortion by a high open loop amplifier gain. However these approaches generally increase both the power consumption of the amplifier circuitry and the size and complexity, and hence cost, of the circuitry.
For many electronic devices, e.g. especially for battery powered devices where power demands impact on battery life, power efficiency of the components of the device may be a concern. Higher power consumption may also have associated cooling requirements. The size and cost of components are also of concern for such applications.